Enveloped viruses such as influenza, parainfluenza, and rhabdoviruses assemble by budding at the plasma membrane of the host cell. During this process, they acquire a membrane whose lipid composition relfects that of the host cell plasma membrane while the protein composition is virus specific. The initial step in virus infection is the attachment of the virus to the cell surface. It is generally agreed that enveloped viruses penetrate the cell either by fusion of the viral envelope with the plasma membrane resulting in he uncoating of the viral nucleocapsid or by phagocytosis of the virus particle and subsequent uncoating of the nucleocapsid in a process probably involving lysozomes. It is proposed to characterized both the attachment and penetration processes using a combination of biochemical, virological, and biophysical techniques. The structural changes induced in the plasma membrane upon enveloped virus adsorption will be investigated and the biochemical basis for this structural change will be examined. The structural events associated with the carbohydrate of the receptors for enveloped virus on the cell surface will be examined during both attachment and penetration. The relationship between the fusion process of parainfluenza viruses and the state of compression of the target lipid membrane of parainfluenza viruses and the state of compression of the target lipid membrane and the role of the protein in the target membrane will be explored. The entry pathways of envelope viruses will be characterized.